Conventionally, a machine having plural axes, such as an NC machine tool, gives an arc instruction as orthogonal straight lines having two axes, draws position feedback data of the straight lines having two axes on a two-dimensional plane, and adjusts a servo system using a shape of this track as an evaluation reference, as disclosed in Japanese Patent Application Unexamined Publication No. 4-177408 and Japanese Patent Application Unexamined Publication No. 2002-120128.
FIG. 1 graphically expresses an example of a configuration of an NC device. In this example, a servo system is adjusted based on a conventional method using two orthogonal axes.
In FIG. 1, an NC device 1 can control a servo motor 2 that drives the X-axis and a servo motor 3 that drives the Y-axis, thereby freely moving a position of a work table 4, fitted to the front end of each axis, within an X-Y plane.
In the present example, a control unit 11 of the NC device 1 gives an arc instruction to an X-axis driving unit 12 that controls the position of the work table 4 in the X-axis direction, and gives an arc instruction, of which phase is deviated by 90 degrees from the phase of the above arc instruction, to a Y-axis driving unit 13 that controls the position of the work table 4 in the Y-axis direction.
The control unit 11 receives position feedback information from the servo motors 2 and 3 and the work table 4, and converts the position feedback information into normal polar coordinates (x=sin θ,2 y=cos θ, 0≦θ<π), thereby calculating an actual move position of the work table 4. In this case, the work table 4 moves along a unit circle (x2+y2=1). The control unit 11 displays the actual move position of the work table 4 on a monitor 5 using a personal computer or the like, by superimposing the actual move position with the instruction arc (unit circle) as the evaluation reference value of the servo system adjustment.
FIG. 2 shows one example of a monitor screen shown in FIG. 1. FIG. 2 shows a superimposition of the evaluation reference value according to the instruction arc, and the position feedback information obtained by actually moving the work table 4 based on the instruction arc. It is clear from FIG. 2 that an error of the servo system occurs mainly due to quadrant projections 21 to 24 attributable to nonlinearities (frictions) before and after a speed polarity changes. The quadrant projections 21 to 24 appear as shape errors when an arc instruction is given to the orthogonal two axes (X-Y axes).
An operator adjusts the servo system so that the quadrant projections 21 to 24 become close to zero (i.e., instruction arc), while watching the drawing track displayed on the monitor 5. Based on this, the operator can easily evaluate processing precision of the machine tool without actually carrying out a work process.
At the time of adjusting a servo system of a single control axis such as a straight line axis and a rotation axis having no adjacent orthogonal axes, a shape error also occurs before and after the speed polarity is changed. However, in this case, because adjacent axes, like the orthogonal two axes which can easily draw an arc, are not present, a visual evaluation standard on which the servo axis is adjusted is not present. Therefore, the servo system cannot be visually adjusted easily.
For the above reason, at the time of adjusting a servo system of a single control axis such as a straight line axis and a rotation axis having no adjacent orthogonal axes, a work process is actually carried out to evaluate the processing precision of the machine tool. When this method is used, the work efficiency of adjusting the single control axis such as a straight line axis and a rotation axis having no adjacent orthogonal axes is extremely decreased, and the adjustment cost increases substantially.